Field of the Invention
The present invention relates to a solid polymer electrolyte fuel cell formed by stacking a membrane electrode assembly and a separator. The membrane electrode assembly includes a pair of electrodes, and an electrolyte membrane interposed between the electrodes.
Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs a membrane electrode assembly (MEA) which includes an anode, a cathode, and an electrolyte membrane interposed between the anode and the cathode. The electrolyte membrane is a solid polymer ion exchange membrane. The membrane electrode assembly and separators sandwiching the membrane electrode assembly make up a power generation unit. In this type of fuel cell, in use, generally, a predetermined number of, e.g., several hundreds of power generation units are stacked together to form a fuel cell stack, e.g., mounted in a vehicle.
In the fuel cell, electricity is generated by electrochemical reactions of a fuel gas (e.g., gas chiefly containing hydrogen) supplied to the anode through a fuel gas flow field and an oxygen-containing gas (e.g., gas chiefly containing oxygen or the air) supplied to the cathode through an oxygen-containing gas flow field.
In the structure, at the time of power generation operation in the fuel cell, water is produced at the cathode, and back diffusion of the water toward the anode may occur. Therefore, condensed water tends to be retained downstream of the oxygen-containing gas flow field and the fuel gas flow field. In an area where the end of the electrode is overlapped with a water retention area, the electrolyte membrane may be swelled significantly to cause cross leakage of the fuel gas to the cathode. Therefore, the electrolyte membrane may be degraded undesirably.
As an attempt to address the problem, a cell unit of a fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2007-234352 is known. This cell unit includes a membrane electrode assembly and a pair of separators made of electrically conductive material, sandwiching the membrane electrode assembly. The membrane electrode assembly includes an electrolyte membrane, a cathode joined to one surface of the electrolyte membrane, and an anode joined to the other surface of the electrolyte membrane. The air is supplied to the cathode, and the fuel is supplied to the anode. An air chamber is formed between the cathode and one of the separators, and a fuel chamber is formed between the anode and the other of the separators.
A throttle section having a reduced sectional area as a fluid passage is provided at the outlet of at least one of the air chamber and the fuel chamber. Therefore, in the air chamber, the air is discharged through the outlet at a higher speed, and water droplets in the water chamber are removed suitably. According to the disclosure, in the cell unit, even if the operating condition is changed, the supply of the air to the cathode is not disturbed easily, and the cell voltage is stabilized.
However, in Japanese Laid-Open Patent Publication No. 2007-234352, since the throttle section is provided at the outlet of at least one of the air chamber and the fuel chamber, the pressure loss is increased at this throttle section, and the efficiency in the fuel cell system is lowered. Further, the structure of the fuel cell system is complicated, and the production cost becomes high.